The invention relates to an apparatus for operating and controlling a free-piston Stirling engine with a displacement piston in a cylinder, said piston separating a hot cylinder space from a cold cylinder space, and a method therefor.
Depending on the type of energy supplied, Stirling engines can be operated as heat engines or refrigerating machines or heat pumps. A comprehensive overview of the prior art as regards Stirling engines is provided by the dissertation of Martin Werdich, which was published in 1990 as a book with the title "Stirling-Maschinen" Stirling engines! by Okobuch Verlag, Staufen bei Freiburg, with the ISBN No. 3-922964-35-4.
All Stirling engines have an enclosed working medium, generally consisting of air, hydrogen or helium. The Stirling engine operates on the following principle: the working medium is pushed backwards and forwards cyclically by a displacement piston from a space at low temperature, via a cooler, regenerator and heater, into a space at a higher temperature level. The pressure differences which arise in this process serve to drive the working piston, which is normally moved with a phase shift of 90.degree. relative to the displacement piston. Driving the displacement piston requires little energy since the pressure on both sides of the piston is virtually the same. The work of the displacement piston here corresponds merely to that required to overcome the gas friction during cyclic flows of the working medium through the heat exchanger and the regenerator. The displacement piston is generally driven by a crankshaft driven by the working piston. In the case of the free-piston engines, the mechanical elements which lead to the outside, such as the connecting rod, pushrod and crankshaft are replaced by internal spring-mass oscillatory systems.
The known free-piston Stirling engines do not have a mechanical connection between the working piston and the displacement piston. The two pistons can move freely.
The springs are often designed as gas-filled springs. From a state of unstable equilibrium, the pistons begin to oscillate and excite one another even in the presence of small temperature changes and the resulting change in pressure.
The main advantages of free-piston engines lie, on the one hand, in their very simple construction--they comprise just two moving parts--and, on the other hand, in the absence of any lateral guiding forces on the piston, and the avoidance of all sealing problems.
The converted energy can, for example, be transmitted to the outside via an electric linear generator.
Despite these undisputed advantages of the free-piston engine, it has not established itself in practice so far.
The reason for this is the difficulty of regulating the oscillation pattern. Specifically in the case of a fluctuating energy supply or variable loading of the working piston, the amplitude of oscillation and frequency varies and cannot be controlled satisfactorily.
The object on which the present invention is based is to manufacture, operate and control a free-piston engine in a simple and cost-effective manner.